The available space for an LED driver has limited many retrofit applications for LED lamps, such as a tubular LED lamp. The traditional LED architecture and driver topology for tubular LEDs requires a driver space much larger than the space that the existing tube dimensions (e.g. a T5 tube) can offer.
A tapped linear driver design is therefore considered as a good alternative solution for applications with space constraints, since it requires much smaller power components and enables the driver size to be minimized. A basic principle of tapped linear driver is that switching LEDs with a flexible number whose forward voltage matches the input voltage in relatively real time.
Tapped linear drivers suffer from some power loss, in particular when the input voltage (e.g. from the mains) exceeds the string voltage of the LEDs being driven. This causes a decrease of the driver efficiency, and thus influences the system efficiency. In order to have higher driver efficiency and less electrical loss, the maximum achievable LED voltage should be as close as possible to the peak of input voltage.
For example, for a 220V input voltage, if a normal white LED with a forward voltage (Vf) of 3.1V is selected, then to providing voltage matching, around 70 LEDs are needed (regardless of the luminous output requirement of the system). In order to improve the power factor, the LED current limit when the input voltage is around its peak value is set larger than the LED current when input voltage is low. The current flowing to the LED strings thus forms a step-wise shape: when one LED string is turned on or off, the LED current immediately rises or falls to reach the new current limit setting. This gives rise to a modulated light output, which gives rise to flicker.
The tapped linear driver generally requires one or more energy storage units to reduce light flicker and a stroboscopic effect due to the zero crossing of the mains input. A valley fill circuit is for example used for this purpose.
The tapped linear driver has the advantages of a very simple circuit, a low cost driver, and low volume solution. However, the driver requires a very high LED count for lighting applications. For example, if the tapped linear driver is for a 220V (RMS) AC supply and using normal 3.1V LEDs, as many as 81 LEDs may be needed which is quite costly.